1. Field of the Invention
The invention concerns a method for operating a rebreather, in which oxygen is metered to the breathing gas, wherein the oxygen content is monitored by at least one oxygen sensor and wherein the one oxygen sensor is tested by flushing with a gas with known oxygen concentration.
2. Description of the Related Art
Open-circuit diving apparatuses are characterized by a supply cylinder of breathing gas, which cylinder is filled with compressed air or another mix of breathing gas and a one-level or two-level pressure reducer, which reduces the pressure of the gas in the cylinder to ambient pressure. The exhaled air is emitted in the water, whereas only a small fraction of the oxygen in the breathing gas as well is really used. Thus at the water surface, about 3% of the inhaled gas is used (25 l breathing minute volume, 0.8 liter used oxygen, at rest), at a depth of for example 20 m, this value amounts due to the with 2 bar increased ambient pressure only a third, that is 1%. Consequently, for a diving operation at 20 m, 100 times more breathing gas must be carried along than what is actually used.
In order to avoid the low efficiency regarding breathing gas usage which is inherent in the system of open-circuit diving apparatuses (SCUBA, compressed air diving apparatuses), semi-closed circuit and fully-closed circuit rebreathers are employed. By these apparatuses, the breathing is done in a loop. The exhaled air is in these apparatuses cleaned from carbon dioxide by means of a carbon dioxide absorber and is again enriched with oxygen. Such apparatuses are further characterized by a one-part or two-part counterlung, which can receive the exhaled gas volumes. With rebreathers the efficiency regarding gas usage can be improved to up to 100%.
The present invention concerns such semi-closed circuit and fully-closed circuit rebreathers and a method for operating theses devices.
Whereas one by open-circuit diving apparatuses normally always inhales a gas with breathable oxygen content, is by semi-closed circuit rebreathers the pO2 in the loop decided by the supplied amount of gas and the metabolism of the diver and is kept at a defined level in electronically controlled fully-closed circuit rebreathers by means of a control circuit (GB 24 045 93 A, U.S. 2003188744 A1, WO 2005/107390 A2). In manually controlled fully-closed circuit rebreathers is the oxygen supply manually set by the diver and thereby the oxygen partial pressure manually adjusted. The oxygen partial pressure of the breathing gas must be within defined limits to be breathable. Commonly is 0.16 bar considered as a lower limit and 1.6 bar as an upper limit. A pO2 below or over these limits is classified as life threatening. Hence it is obvious, that a constant monitoring of the pO2 is necessary for rebreathers. Fully-closed circuit rebreathers need pO2 sensors for manual or electronically controlled adjustment of pO2 in the loop. Normally electro-chemical sensors are employed as pO2 sensors, which are calibrated with air or 100% O2 before the diving operation at the water surface.
A correctly working pO2 sensor for the application in rebreathers discloses an output signal (current or voltage), which is linearly dependent only on the pO2 before the membrane of the sensor.
pO2 sensors are very prone to errors. Typical errors which might occur are:                a) nonlinearity;        b) current limitation: in this case the pO2 sensor becomes nonlinear above a certain pO2 since the output current of the sensor (or the output voltage) due to an error can not rise over a certain level. This results in too low sensor signals for high pO2;        c) erroneous signals from one or more sensors respectively the sensor signal processing;        d) erroneous calibration,        
pO2 measuring equipments are, as already mentioned, calibrated at the water surface with air or 100% O2 under normobaric conditions (at sea level therefore ˜1000 mbar ambient pressure), whereby the sensitivity of the sensors is decided. The maximally reachable pO2 is thus 1.0 bar. Since during diving operations there often is a pO2 higher than 1.0 bar, it is important to test the sensors for a) and b). (Example for a calibration with 100% O2: ambient pressure 1000 mbar, output voltage signal: 50 mV→sensitivity=50 mV/bar pO2)
One tries to counter the error susceptibility of the pO2 sensors by redundant use of pO2 sensors. Hence normally three oxygen sensors are used in fully-closed circuit rebreathers. In case a sensor drops out, and thus its output signal differs from that of the other two, is this detected through a comparison between all three sensor signals with a “voting algorithm” (GB 240 45 93 A, WO 2004/112905 A1), and this sensor is no longer consulted for the adjustment of the pO2.
In that way an erroneous sensor can be identified. This method however fails to work for the following errors:                e) drop-out of two sensors, which yet have the same output signal;        f) the same non-linearity for at least two sensors (>=two sensors from the same production batch, the same age, the same conditions . . . );        g) the same current limitation for at least two sensors.        
Furthermore, for a detailed diving analysis, a continuous recording of all diving relevant data is necessary. Thus, depth profile, time and pO2 are often stored in an internal memory of the pO2 measuring equipment and can be transferred to a personal computer after the diving operation, wherein the resolution of time and the maximal length of the recording depends on the size of the memory and is thus limited.
Normally, special interface cables are needed for transfer to the personal computer. Especially for the efficient treatment of diving accidents, a fast analysis of the diving data is important. However, the fitting interface cable is often not available on the spot. The possibility to read such diving data without such a special cable with every commercially available pc is therefore desirable.
An object of the invention is therefore to develop a pO2 measuring equipment in such a way, that errors in the pO2 sensor signals, nonlinearities of the pO2 sensor signals, a possible current limitation of pO2 sensors are reliably detected and a detailed recording of the relevant diving data is made possible.
From U.S. Pat. No. 4,939,647 A a method is known, which at least partly solves the above-mentioned problems. Thereby an oxygen sensor is calibrated through flushing with pure oxygen. This makes a calibration at an oxygen partial pressure of 1 bar possible. However, it has turned out that such a calibration is not sufficient to reliably detect the above-mentioned errors.